I’m a little confused to how an airplane wing produce lift. I heard there is a Bernouli principle that says faster fluid equates to lower pressure, and that the air above the wing is faster than below so the pressure difference produce lift.
But what makes the air above wing go faster? Is it to due to the shape/profile of the wing? Or is it because of the angle of attack deflects air downward which makes them slower?
Then again, is lift produced because of the change in momentum of the deflected air molecules below the wing (due to Newton’s third law)? Or is it simply because the air molecule gets deflected and become slower and therefore produce lift due to Bernouli’s principle? Or both? Or are they both the same thing (mathematically/physically equivalent)?
Look at a cross section of a wing. Notice the distance along the top is longer than the distance along the bottom. Air is a fluid. The air under the wing basically moves straight. The air on top has farther to go, thus has to move faster, reducing pressure on top of the wing and producing lift.
You may be able to experience this first hand. While riding in an automobile, hold your hand out the window, perfectly flat and parallel to the road. Now cup your hand slightly—it will go up. It is easier to do something like this in a denser fluid like water, if you have the opportunity.
There are so many factors w/lift & aircraft to consider. Four main things are lift, drag, thrust, and weight. Lift is created (most commonly) by an airfoil. An airfoil is a wing.
Like Triarius said a cross section will show the shape of an airfoil. Air starts in front of the wing, where it is “split” by the leading edge, the bottom side of the air will continue on straight (in a simple airfoil) path below the wing, while at the same time, the air above the wing is sped up to keep up with the bottom. This creates a lower pressure above the wing, and higher pressure below, creating lift.
The curve of the leading edge to the trailing edge is called camber. Camber can be increased by the addition of flaps, which generate more lift and drag, and are used primarily for short take off and landings, as well as slow flight and normal landings.
If you have any questions, or if anybody wants to correct me, please post.
Yeah,
Ross and the Cowboy have it right.
Bernoulli’s principle states that in fluid flow (and air can behave like a fluid) , an increase in velocity occurs simultaneously with decrease in pressure. This is a fairly serious simplification of a very complex physical law - but in essence, the longer distance (and therefore higher velocity) the air has to travel over the upper surface of an aerofoil causes a lower pressure area on the wing upper surface (much lower than the pressure under the wing) - causing lift.
A simple example of this is the effect of placing a straw in a glass of water - and then blow across the opening at the top of the straw. As the increased velocity at the opening decreases the air pressure at that point, it also decreases the pressure inside the straw, and the pressure differential between the fluid in the glass and the air in the straw forces the fluid upwards, above the level of the fluid in the glass.
Same goes for cars - like race cars. Turn that aerofoil upside down and you have a “downforce element”.
Theoretically, a Champ Car should be able to drive at full speed on the ceiling of a tunnel, as the calculated downforce at speed actually exceeds the weight of the car.
Simple, right?
Well, the low pressure/high pressure thing isn’t the whole story. The low pressure area on the top of a wing isn’t strong enough to lift the airplane of itself, i.e. the wing isn’t sucked up into the low pressure.
What really happens is the pressure differential deflects the airflow in proportion to the angle of attack, so you are constantly accelerating a mass of air in a “down” direction, and as long as the mass of the air is equal to the mass of the airplane, you get steady state flight. By accelerating more air, you can climb, accelerate less air and you descend. The air deflection is dependant upon a smooth flow of air over the wing, so when the angle of attack gets too high, the efficiency of the flow is lessened and becomes turbulent, so the wing stalls. Same thing with contaminated surfaces.
So, you are correct that lift is Newtonian (action/reaction), enabled by Bernouli (fluid dynamics), all made possible by money!
One minor point…air IS a fluid. The definition of a fluid, according to “Introduction to Fluid Mechanics”, 4th ed., Robert W. Fox & Alan T. McDonald, is as follows:
fluid: a fluid is a substance that deforms continuously under the application of a shear (tangential) stress no matter how small the shear stress may be.
This was the first thing hammered home by the professor in this class…“fluid” is usually assumed (in layman’s terms) to mean the liquid phase of a substance.
Next time you are riding in a car, roll down the window. Stick your arm out- not to far so that it gets cut off, etc. That would be bad. (Unless you have spare arms readily available…)
Hold your hand out flat, thumb forward. Your hand should slice nicely through the air. Now, tilt it back a little, like the angle on an airplanes wing. Ah-ha! Lift!
The same principle can demonstrate a planes elevators. Tilt it one way, your hand goes up. The other, your hand goes down.
You can get fancy and try cupping your hand, or dropping your thumb lower, or lowering your pinky and ring finger (flaps…).
Now, if you really want to take it a step further, build a slim, elipsed, wing-shaped glove, and ride in a car at 360 miles per hour with your hand sticking out. Keep shouting “I’m a Spitfire! I’m a Spitfire!” LOL
And then you can take your time to write about your observations while you convalesce in the hospital because you’re are playing airplane and not paying attention to your driving. [;)]
The short answer is that lift is a particuarly difficult force to explain. The How Stuff Works web site on airplanes really helps, but it’s basic. However it does expose the problems with the commonly accepted Bernoulli and Netownian explanations. It also goes into why symetrical airfoils still provide lift.
A more in-depth explanation of aerodynamic in, and how it affects flight is on the web site: See How it Flies. In particular, check out chapter 3 on airfoils.
“The speed of air is inversely proportional to the volume.” This means that the faster air goes, the thinner it gets … so it stretches out and that creates a partial vacuum … a low pressure area.
The curve on the upper side of the wing forces the air on top to go a longer distance than the flat bottom side, so it stretches out and speeds up to get to the trailing edge, creating a vacuum that sucks the wing upward.
The angle of attack, or tilt of the wing into the airflow, increases the vacuum effect until it reaches a point called the “stall” where turbulence and drag overcome lift and the wing falls.
These basic aerodynamics are managed and altered with flaps, leading edge slats, tilting wings and variable angles of sweep forward or backward, but the basic principle is the cambered airfoil.
All the posts above have said this in one way or another … this is just my version of the same thing.
So let me ask two additional questions for you all to ponder:
How does a “molecule of air” “know” that it is flowing over the top of the wing instead of the bottom of the wing?
How is it possible for an airplane to fly upside down and still not lose altitude?
These questions seem to defy the “Bernoulli Theorem” explanation of lift and stump most people who think they know all about how an airplane wing works. The explanation actually lies in the “Circulation Theory of Lift” which I’ll leave you to Google on your own.
Read the two links that I’ve posted above. They answer your questions very well; better than I could in this post. Seriously, both links ask and answer the exact same questions.
The bottom line is that Bernoulli was only partly right. For all intents and purposes, air does not behave like a bunch of molecules, but as a fluid. There is a rotational effect around the wing (front to back, underneath toward the front again) that creates lift.
The air molecules don’t have to know whether they’re going over the top of bottom of the wing. Remember, the wing is slicing through the air, so as the wing sliced through, the air of forced to flow over and under the wing. The air on the upper side reaches the back of the wing at the same time the air on the bottom side does, but because of the airfoil shape of the wing, the air on the upper surfaces has to travel faster relative to the air on the bottom. So the “relative speed” of the upper air is faster.
Simple physics. Air striking the leading edge of a wing or any pbject, has a tendency to rejoing where it is seperated. As has been memtioned, the shape of the top of the wing causes the air to travel faster to enable it to rejoin the air under the wing. It does not create a vacuum at the trailing edge of the wing. If it did, it would cause problems with back flow, which is a form of drag. Run your hand through water real fast. Does it leave a vacuum in the water behind your hand? No, same theory. Water is fluid just as air is fluid.
An aircraft needs two things to fly. Lift and thrust. Lift must overcome gravity and thrust must overcome drag.
How does an aircraft fly inverted. Simple, by increasing the angle of attack (AOA) of the wing to increase air speed going over the wing. The pilot just pushes forward on the stick while inverted to increase his AOA. The nose is actually pointing above the direction of flight.
Great answers so far, but I think Berny13 hit on a important part of the answer. “Thrust”, even sticking your hand out the window of a moving car is an excellent example. The car is providing the thrust. Although not aerodynamic, a 6lb. rectangular brick will fly with enough thrust behind it.
This doesn’t explain a lot but is a really cool demonstration of lift.
1 Turn on you tap over the kitchen sink.
2 Get a teaspoon.
3 Hold the teaspoon vetically, gently between your thumb and forefinger.
4 Slowly move the teaspoon till the back of the spoon is touching the flow of water.
There you are. A very simple and effective demonstration of the airflow over an aerofoil. Pay special attention to how the water continues after it leaves the trailing edge. This has more to do with the way a wing works than you would expect. It’s very complicated and well outside my ability to explain.
This won’t help answer any questions but if you don’t have access to a wind tunnel and somke generator then is a very cheap second.
This is why it has always been called the -THEORY OF FLIGHT!!!
Since everyone has a some what corect version of what happens, let’s not beat a dead wing here.
